The present invention relates to a communication system and more particularly to a system for detecting and identifying one or more of a plurality of independent tag circuits whenever such circuits come within range of a monitoring station. It also relates to the tag circuits per se.
There exist numerous applications for a communication system capable of keeping track of the coming and going and whereabouts of a particular object, whether it be animate or inanimate. Heretofore, systems have been developed for keeping track of vehicles where the vehicles travel comparatively known paths past fixed installations. For example, in U.S. Pat. No. 3,859,624, issued Jan. 7, 1975 to Kriofsky et al., there is described an interrogator-responder arrangement which, according to the patent abstract, employs a responder tag that receives power as the vehicle carrying it approaches the interrogator unit. The power is received through inductive coupling and the responder tag is stated to generate a uniquely coded information field unique to the particular responder tag. The coded information field is received by the interrogator and converted to an information signal representative of the tag. Where the tag is self-powered, the interrogator means does not generate an AC power field and the inductive coupling between tag and interrogator is limited to the coded information field. According to the patent disclosure, the responder tag either operates continuously if self-energized, or whenever it receives transmitted power from an interrogator station. The system utilizes digital techniques and employs a message format for response that consists of 32 bits, the first seven of which represents a synchronization pattern. This is followed by a parity bit and then six groups of 4 bits each in binary coded decimal form providing a 6-digit identification number. The responder tag is held to the barest essentials and only the interrogator is provided with means for validating the signal received from the other.
Responder tags have also been used as part of a credential system for identifying authorized personnel and monitoring passage into or out of a controlled area. A system for that purpose is described in a project report entitled "The Self-Energized Credential System for the Plutonium Protection System" by Thurlow W. H. Caffey and David E. Barnes available from the U.S. Department of Energy, P.0. Box 62, Oak Ridge, Tenn. 37830 under document No. SAND78-2156, printed Dec. 1978. In this system a portal loop transmits a continuous tone at 110 kHz, and the credential transmits an identification code in bursts of a 55 kHz tone. The 55 kHz tone burst is amplified, converted to binary form in the decoder, and tested to see if certain format and parity conditions are met. If the binary code is validated, it is made available for transmission to an operations center. The particular code format incorporates a group of four hexadecimal digits grouped as two pairs with three sync bits and a parity bit added to each pair. The entire code word is 24 bits long. The first digit in each hexadecimal pair is fixed as 1 and 0, respectively, so the first hexadecimal digit is always eight or greater and the third hexadecimal digit does not exceed seven. Because of this coding format and the method used for validation, the number of unambiguous available codes is only 16,192 out of a possible 65,536.
Neither of the systems mentioned above is able to handle the simultaneous passage of more than one responder tag past an interrogator point. This limitation can be tolerated in situations where it is either natural or expected that passage past the control point will be in single file. However, there are many situations where a monitoring function is desired with minimum, if any, intrusion on the normal movement of the carrier. For example, in a hospital it would be extremely useful to be able to know at all times the whereabouts of doctors, nurses and other personnel to facilitate communication with any desired individual. Another purpose of knowing the whereabouts of personnel might be for record purposes to be able to establish when particular personnel were in attendance upon a particular patient or were in an operating room or the pharmacy. The number of uses are limited only by the imagination.
However, where such a system is to be used for documenting whereabouts or for important control it is essential that it reliably detect responder tags passing a monitoring station even though more than one tag might come within range of the station at the same time. The present invention provides just such a system. Moreover, as will appear from the ensuing description, the system in its present embodiment provides for monitoring 65,536 tags through a plurality of portals. It will reliably handle up to 6 tags simultaneously through any one portal.